This PR add a feature gates mechanisim to client-go
as described in https://docs.google.com/document/d/1g9BGCRw-7ucUxO6OtCWbb3lfzUGA_uU9178wLdXAIfs
In particular:
- Adds a default feature gate implementation based on environment variables.
- Adds a set of methods for reading, overwriting the default implementation, and adding features to an external registry.
Co-authored-by: deads2k <deads@redhat.com>
Co-authored-by: Ben Luddy <bluddy@redhat.com>
Kubernetes-commit: d74c57d4f592d20a992afb54b1ee64f56215210e
